Study On The Microstructure And Properties Of Rare Earth Micro-alloyed High Carbon Bainite Steel

Rare earth elements possess high chemical reactivity and can be added to steel to play a purifying role in deoxidizing and desulfurizing,controlling inclusion modification,and strong microalloying.This study focuses on the rare earth microalloying of high carbon bainitic steel,and conducts performance tests such as tensile,impact,and wear resistance.The phase structure,microscopic structure,fracture surfaces,and wear resistance properties are characterized using scanning electron microscopy,transmission electron microscopy,X-ray diffraction,EPMA,and three-dimensional profilometry.The influence mechanism of rare earth elements on inclusions,bainite structure,bainite phase transformation,and mechanical properties in high carbon bainitic steel is studied.The results of the study on the effects of different heat treatment processes on the structure and tensile properties of rare earth micro-alloyed high-carbon bainitic steel show that there are no obvious large unsolved carbides in the structure obtained at the austenitizing temperature of 950 ℃.The structure size and hardness performance are in the middle,and the carbon content in the residual austenite is the highest,making it the best austenitizing temperature.Under the same heat treatment process,the addition of rare earth elements has a slight inhibitory effect on grain growth,but has almost no effect on the size of bainite packets,bainite transformation completion rate,and tensile properties of the steel,which is related to the lack of rare earth content dissolved in the steel.As the bainite isothermal temperature decreases from 300 ℃ to 220 ℃,the hardness,tensile strength,and yield strength of rare earth micro-alloyed high-carbon bainitic steel increase,but the elongation significantly decreases.The research results on the influence of different rare earth contents on the microstructure and properties of rare earth micro-alloyed high carbon bainite steel show that Ce can form spherical oxide and phosphide inclusions by combining with C,O and P atoms in the steel,improving the morphology of inclusions.Meanwhile,with the increase of Ce content,the content of residual austenite decreases,but it has little effect on the carbon content in the residual austenite.The addition of Ce improves the impact performance of rare earth micro-alloyed high carbon bainite steel to a certain extent,but as its content continues to increase,the size and irregular shape of inclusions increase,which in turn reduces the impact performance of rare earth micro-alloyed high carbon bainite steel.The study on the influence of cryogenic treatment on the microstructure and properties of rare earth micro-alloyed bainitic steel shows that the sample after bainitic isothermal salt bath + tempering treatment(SBT)consists of needle-like bainitic ferrite and residual austenite,while the sample after bainitic isothermal salt bath + deep cryogenic treatment +tempering treatment(DCT)contains a small amount of martensite in addition to bainitic ferrite and residual austenite.Compared with the SBT sample,the DCT sample has higher hardness,better wear resistance,and less loss of impact toughness.When the contact pressure of wear is low,both samples have many adhering blocks and a large number of furrows on the worn surface,and the wear mechanism is mainly adhesive wear and abrasive wear.With the increase of contact pressure,the wear mechanism of both samples shifts to oxidative wear.The study on the influence of carbides on wear resistance of rare earth micro-alloyed high carbon bainite steel shows that the hardness and residual austenite content of bainite with unsolved carbides obtained by austenitizing and holding at 860 ℃ is comparable to that of bainite without carbides obtained at 1000 ℃,but the carbon content in the residual austenite of bainite without carbides is higher.Under dry friction conditions,the bainite without carbides has higher wear resistance,especially under high stress,compared to the structure with carbides precipitated.Under oil lubricated friction conditions,the friction coefficient is greatly reduced and the wear resistance of both is comparable.Unsolved carbides in the structure will peel off during the dry friction wear process,and are susceptible to oxidative wear,resulting in delamination damage,thereby aggravating the wear degree,which is detrimental to wear resistance.